USING Ce TO MODIFY INCLUSION IN SPRING STEEL

Q. Wanga,b, L.J. Wanga,b*, Y.Q. Liuc, K.C. Choua,b

a* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing,Chinab Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing,China

c Shougang Jingtang United Iron & Steel Co..Ltd, He Bei Province, China

(Received 17 November 2016; accepted 24 April 2017)Abstract

The effect of rare earth metals addition on the Al2O3 inclusions in spring steel used in fastener of high speed railway wasinvestigated by metallographic examination, SEM-EDS and composition analysis. To deform those harmful inclusions toimprove material performance，the evolution process of Al2O3 inclusions was investigated through the surface and linescanning. Ce metal modifying Al2O3 is a stepwise reduction process based on a formation of ring shape Ce-riched bandaround the Al2O3 inclusions during reaction process. Through experiment and thermodynamic calculation, an evolvementrule about Al2O3 inclusions change after Ce addition is obtained, i.e. Al2O3→Ce2O3-Al2O3→Ce2O2S. Changing theinclusions from Al2O3 to rare earth inclusions could improve the resistance to pitting corrosion based on potentiodynamicanodic polarization test.

Keywords: Cerium; Inclusions; Thermodynamic calculation; Al2O3; Spring steel

*Corresponding author: lijunwang@ustb.edu.cn

Journal of Mining and Metal lurgy,Section B: Metal lurgy

DOI:10.2298/JMMB161117026W

1. Introduction

Al2O3 inclusions are often observed in Al-killedsteel as the deoxidation product. Such inclusions arecommonly considered to be very harmful to thequality of the final products, acting as the mainsources of pitting corrosion [1] as well as the origin ofmicro-cracks and causing the clogging of submergedentry nozzle during continuous casting due to theirhigh hardness and high melting temperature [2].X.Zhang indicates that crevice can easily form aroundthe Al2O3 particles when the inclusions grow up to acertain size，so that large Al2O3 inclusions is easy tobe the origin of pitting corrosion when theenvironment is full of Cl- [3]. The technique of rareearth metal addition into steel is sometimes used dueto the beneficial effects of their use in steels. In recentyears, Al2O3 inclusions have been widely investigated,and most of the reports mainly concentrated oncalcium treatment to modify Al2O3 inclusions in steel[4-8], while the effect of rare earth element, such asCe, on the modification of Al2O3, is rarely involved.

A considerable amount of researches have beenconducted on predicting the thermodynamicbehaviors of rare earths in steelmaking during the pastseveral decades [9-11]. S.Diao [12] determined thestandard free energy of formation of CeAlO3 and

NdAlO3 in steel melt. G.Li and H.Suito [13]determined that the effect of Ce on the supersaturation behavior of the precipitation of Al2O3.A.Katsumata [14] studied large cluster inclusions byAl-Ce complex deoxidation in steel and found thatdeoxidation products have Al2O3, CeAlO3 and Ce2O3and S.K. Kwon [15] also found the similarphenomenon in the formation behavior of inclusionsin Ce-containing stainless steel. LJ.Wang [16]indicate the mechanism of dissolution of Al2O3 in theCe2O3 containing slag and it could propose as threesteps involved: 1) the formation of calciumaluminates CaO•Al2O3 at the interface 2) theformation of 3CaO5Al2O3Ce2O3 as the reactionprogresses; and 3) the dissolution of3CaO5Al2O3Ce2O3 into the slag. Several researchershave reported that the rare earth oxysulfides are foundmore often than the sulfides or oxides [17-18], but theeffect of S content is little involved in the inclusionschanging process.

SH. Jeon et al [19] point out that the addition of Ceto the HDSS increases the resistance to pitting corrosionbecause of the formation of stable Ce oxide and adecrease of pit initiation sites. It is certainly worthwhilefor industrial purposes if the pitting-corrosion resistanceof spring steel can be improved through the addition ofrare earth metals to the spring steel. Thus, it would be

J. Min. Metall. Sect. B-Metall. 53 (3) B (2017) 365 - 372

Dedicated to the memory of Professor Dragana Živković

Q. Wang et al. / JMM 53 (3) B (2017) 365 - 372

useful to carry out Ce treatment to partially evolution ofAl2O3 to reduce the possibility of pitting corrosion.

In this paper, the formation of Al2O3 inclusions inspring steel used in fastener of high speed railway at 1873K is discussed in detail. The effect of Ce metal additionon a change of inclusion chemistry was discussed. Inaddition two thermodynamic phase stability diagram ofthe inclusions were calculated by FactSage6.4 software.An investigation has been made on the influence of theinclusion types on the resistance to pitting corrosion.

2. Experimental procedure

All the raw materials including the aluminum areadded into a high frequency vacuum inductionfurnace to obtain the initial spring steel and then castinto ingot under an Ar atmosphere. Table 1 shows thechemical compositions of the experimental steel.

2.1. Inclusion modificationThe ingot was cut to appropriate block by wire

cutting. In a typical run, 180g of spring steel werecontained in an MgO crucible (ID of 30 mm, OD of40 mm, and height of 85 mm) and placed in a verticalMoSi2 resistance furnace. The temperature of theliquid steel was controlled in 1873K under high-purityAr atmosphere. In order to protect Ce metal (99.9%,aladdin) from oxidation, the Ce metal was wrapped inreduced iron powder and pressed into a metalcylinder, then added the metal cylinder wrapped inpure iron chips to the molten steel and stirred themolten steel by using a long quartz tube. It has goodeffect on Ce homogeneity in the molten steel. Theaddition of Ce metal is based on the experimentalprogram which is showed in Table 2. After 5 min forsteel-inclusion reaction, the crucible was rapidlyquenched by water from the furnace. All tests in ourexperiment have the same experimental manipulationincluding cooling treatment. The only difference is thecontent of Ce addition. Thus, we consider the differentinclusions caused by Ce content in molten steelinstead of cooling rate.

2.2. Analysis

To observe the inclusions in the spring steel, themetallographic sample(10mm×10mm×10mm） were

obtained from treated steel center and ground to 2000grit using SiC abrasive papers, then polished withdiamond paste. The chemical compositions andmorphology of inclusions were analyzed through ascanning electron microscope (SEM) and an energydispersive spectroscope (EDS). Each sample wasobserved with 10 view fields under magnification 500times (0.034 mm2)， and approximately 100inclusions were observed which depends on thecleanness of the steel sample. The chemicalcomposition of steel samples was analyzed throughICP-AAS or ICP-MS. The oxygen activity in steelmelt was measured by an electro motive force method(EMF）at 1873K.

2.3. Corrosion test

Three kinds of spring steel sampled from 1#, 2#and 6# test were used to investigate the effect ofinclusion types on the resistance to pitting corrosion,and it were analyzed by a potentiodynamic anodicpolarization test in the deaerated 3.56wt% NaClsolution at 298 K by applying the method defined bythe ASTMG61. The test samples were welded withcopper wire through solder and then embedded by theepoxy resin. The side of sample used in this test waspolished with 600-grit SiC paper until previous coarsescratches were removed, then rinsed and dried. Theexposed area of test samples was 1cm2.

3. Result and discussion 3.1. Effect of Ce addition on spring steel

chemical compositions

The chemical composition of each sample isconducted, and the results are showed in Table 3. Ascan be found, the alumina and magnesium contentsvaried in the range of 0.051～0.054% and 0.0001～0.0002%, respectively. In addition, the yield of Cedecreased with the increasing of theoretical addition.The oxygen activity, total oxygen contents and sulfurcontents varied in the range of 0.000018～0.000009,0.005%～ 0.0012% and 0.0042%～ 0.0025%,respectively. The addition of Ce decreased the oxygenactivity, total oxygen contents and sulfur contents insteel melt.

366

Elements C Si Mn Cr Ni V TiContent 0.45 1.9 0.7 0.7 0.9 0.075 0.035

Table 1. Chemical composition (wt %) of the spring steel

Table 2. Experimental program

No 1# 2# 3# 4# 5# 6#Steel/g 180 180 180 180 180 180

Ce addition /% 0 0.007 0.014 0.028 0.06 0.12

Table 3. The analysis results of chemical composition(wt%)

Elements 1# 2# 3# 4# 5# 6#Ce 0 0.006 0.011 0.017 0.034 0.056Al 0.051 0.053 0.052 0.056 0.052 0.054Mg 0.0002 0.0001 <0.0001 <0.0001 0.0001 0.0001[O] 0.0018 0.0017 0.0016 0.0014 0.0011 0.0009S 0.0042 0.0041 0.0038 0.0035 0.003 0.0025

T.O 0.005 0.0038 0.0031 0.0026 0.002 0.0012

3.2 Evolution of inclusion in spring steel afterCe treatment

3.2.1. Effect of the Ce addition on thecharacteristics of inclusions

The typical inclusions in 1# to 6# steel samplewere observed through SEM and the chemicalcomposition of inclusions were analyzed by EDSattached to the SEM, as shown in Fig. 1 and Table 4.From the statistical results, it was known that theinclusions in 1# steel sample were mainly composedof Al2O3 type after alumina deoxidation. In addition,clusters of Al2O3 inclusions with the angular shapewere observed in the present study. The Al2O3inclusions wrapped by rare earth inclusions wereobserved in the 2# and 3# sample. With the increasingof Ce content in steel melt, inclusions in 1# steel werecompletely modified to rare earth oxysulfidesinclusions in 4# steel sample ([Ce%]>0.017). WhenCe content further increases, the inclusions in 5# and6# sample were still rare earth oxysulfides. Inaddition, the shape of the inclusions became globularor spheroid.

The inclusion size distribution and average sizeare shown in Fig. 2. The large inclusion distributionand inclusion average size decreased with theincreasing content of Ce in the present results. Itindicates that the addition content of Ce plays animportant role in the size distribution of inclusions.

From these findings it could be concluded that therewould be an optimal addition content of Ce in whichinclusions were refined. Therefore, the appropriateaddition of Ce could contribute to producing the fineand dispersive rare earth inclusions.

3.2.2 Effect of Ce addition on the evolution of Al2O3inclusions

There are three main roles of rare earths in steels,which are modification of inclusions, deepdeoxidation and desulphurization, alloying. Asdescribed above, modification of inclusions , such asMgO•Al2O3 and Al2O3，can be achieved through

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Figure 1. SEM images of typical inclusions in 1# to 6# steelsample

Table 4. EDS analysis of typical inclusions in 1# to 6# steel sample

wt% Al Ce O S Fe (matrix)

1# 54.41±3.21 45.59±4.21 Al2O3

2#A 34.82±1.81 17.26±1.14 33.23±2.33 3.45±0.33 11.24±0.89 Al11O18Ce+ Ce2O2S

2#B 51.24±3.89 5.46±0.41 40.85±4.42 2.45±0.15 Al2O3

3# 10.34±1.02 53.62±4.31 20.78±3.14 15.26±1.24 AlCeO3

4# 82.21±6.25 17.79±2.31 Ce2O3

5# 75.99±4.32 10.53±1.21 3.46±0.31 10.02±1.16 Ce2O2S

6# 86.98±4.16 5.19±0.89 7.83±0.85 Ce2O2S

Figure 2. Changes of inclusion size distribution andaverage size

adding the Ce metal to the steel melt. Ce plays animportant role on the shape control and themodification of inclusions in spring steels.

Fig. 3 shows the SEM-Mappings of the Ce2O3-Al2O3 or Ce2O2S-Al2O3 system inclusions. Fig. 3(a)and Fig. 3(b) are the typical inclusions in test 2 andtest 3 after Ce treatment, respectively. When the Cealloy added into the steel melt at 1873K, Cecompounds correlating with O began to aggregate andreact with Al2O3 inclusions. It was determined thatshared features of Ce2O3-Al2O3 system inclusionswere high Al content in the inner part while the highCe content in the outer surface. And almost no Sdistributed in the inclusions. Moreover, theconcentration of Ce was much higher in the outersurface of inclusion while much less in the inner partforming a ring shape Ce-riched band around theinclusion in the SEM-mapping image, as shown inFig. 3(a). The Al2O3 inclusions were wrapped by theinclusions containing Ce content. However, there

were some differences between SEM-mappings ofinclusions in Fig. 3(a) and that of inclusions in Fig.3(b). It was found that common features of Ceinclusions were high Ce content in the wholeinclusions except for a very small region in the innerpart. Concentration of Al in the inner part ofinclusions in Fig. 3(b) was lower than that in theinclusions in Fig. 3(a). And a high S content regionwas found in the outer surface. Particularly, Ceconcentration zone and Al concentration zone seemedto be incompatible and complementary in the SEM-Mapping images, which was more obvious in Fig. 3(a).

Therefore, it is believed that rare earths inclusionscan transfer from high melting temperature Al2O3inclusions, which also has been proved by SEM linescanning results.

SEM line scanning results of inclusion in test 2was shown in Fig. 4. As can be seen in Fig. 4, theinclusions core was mainly composed of Al2O3

Q. Wang et al. / JMM 53 (3) B (2017) 365 - 372 368

Figure 3. SEM-mappings of transferring inclusions

content that changed along width direction and Ce hada high content at the edge of inclusion. Nevertheless,S content was low and changed little along the widthdirection. It is concluded that the inclusion is beingchanged from Al2O3→Ce2O3-Al2O3→Ce2O2S.

Based on the above SEM-mapping and linescanning results of inclusions, it was summarized thatthe high melting temperature and solid Al2O3inclusions wrapping by rare earths inclusions layerwas the common feature of all Ce2O3-Al2O3 orCe2O2S-Al2O3 system inclusions. At last, the shape ofinclusions evolved from irregular into spherical rareearths inclusions with the increase of [Ce] content.

3.3. Thermodynamic calculation on evolution ofinclusions

The stability diagrams of Ce-O and Ce-S werecalculated by using FactSage6.4 software(Datebases:FToxid, FactPS, FTmisc, Phase Diagrammodule) based on the spring steel chemicalcomposition at 1873 K. The results were shown inFig. 5.

In Fig. 5(a), the inclusions could change in theorder of “Al2O3→ Al11O18Ce→ AlCeO3→ Ce2O2S orCe2O2S+Ce2O3” by increasing the content of Ce atoxygen content from about 0 to 0.02 %. The Ce2O2Sis the most stable inclusion phase by increasing thecontent of Ce at oxygen content less than 0.004 % inspring steel. In Fig. 5(b), the inclusions could changein the order of “Al2O3→ Al11O18Ce→ AlCeO3→Ce2O2S” by increasing the content of Ce at S content

from about 0.002 % to 0.01 %. The Ce2O3 inclusioncould generate by increasing the content of Ce at Scontent less than 20 ppm in spring steel. Ce2S3 andCe3S4 inclusions could form by increasing the contentof Ce at S content more than 0.01 % in spring steel.

Fig.5(a) and Fig.5(b) shows the data pointscorresponding to the experimental [Ce]-[O] and [Ce]-[S] contents in steel samples (5 min), respectively. Itcan be obtained from Fig. 1-5 and Table 3-4 that theinclusion variation is Al2O3→ Al11O18Ce→ AlCeO3→Ce2O2S in present spring steel at 1873K and Ce2O2Srepresent the stable inclusion phase in present moltenspring steel, which a similar evolution processbetween thermodynamic calculation and experimentresults. Therefore, Ce metal modifying Al2O3 is astepwise reduction process.

3.4. Illustration on evolution of inclusions afterCe treatment in spring steel

Through the above results of experimental dataand thermodynamic calculations, the evolutionprocess of inclusions had been known. To furtherunderstand the evolution process of inclusions, it isnecessary to illustrate the process about inclusionstransformation. The change process of inclusions wasclearly described by Fig. 6 and illustrated asfollowing:

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Figure 4. SEM line scanning of transferring inclusions

Figure 5. Phase Diagram of Ce-O and Ce-S in moltenspring steel

(1) Al2O3 inclusions and clusters were quicklyformed in steel melt in a few minutes after Aldeoxidization adding to steel melt, as shown in Fig. 6(a).

(2) The formed Al2O3 was not stable when Ce wasadded into steel melt. With the process of reaction,[Al] would be reduced from Al2O3 inclusions bychemical reaction Eq. (1) and (2). As a result, theAl2O3 inclusions would be transferred into Al11O8Ceand AlCeO3 inclusions as presented in Fig. 6(b).However, Fig. 6(b) is shown the intermediate state ofinclusion modification process. The inclusionmodification process is fitted to the unreacted coreshrinking model. Thus, the new formation phase isdistributed in surface of inclusion, which would makethe inclusion heterogeneous.

(1)(2)

(3) With the increasing of Ce addition, the AlCeO3would be transferred into Ce2O2S by chemicalreaction Eq. (3). As shown in Fig. 6(c), all of Al2O3inclusions in steel melt were completely modified torare earth inclusions. And the distribution of rare earthinclusions was uniform in size and spherical inmorphology. The content of [S] in steel melt wasdecreased with Ce additions.

(3)

3.5 Effect of inclusion type on the resistance topitting corrosion

In general, the pitting potential (Ep) is defined asthe breakdown potential destroying a passive film. Asthe Ep of an alloy increases, the resistance to pittingcorrosion of the alloy increases [20]. Those three steelsamples used in potentiodynamic anodic polarizationtest represent the spring steel contained Al2O3, Al2O3wrapped by rare earth inclusions and rare earthinclusions, respectively. Fig. 7 shows the result ofpotentiodynamic anodic polarization test for 1#, 2#and 6# steel samples. Table 5 shows the variation ofresistance to pitting corrosion with the increasing Cecontent. With the inclusions modified from Al2O3 torare earth inclusions, the breakdown potential shows

the trend of increasing. It shows that the pittingpotential (Ep) of the 6# steel is the most stable, whichindicate Ce2O2S is helpful to improve the resistance topitting corrosion of spring steel. Adding a smallamount of Ce metal which would partly modifiedAl2O3 could also improve the resistance to pittingcorrosion in spring steel. Fig. 8 shows SEM images ofthe pitting sites on the samples after the potentiostaticmeasurements. Fig. 8(a) shown the serious pittingcorrosion occurred around the Al2O3 inclusion with adiameter more than 20µm which is much larger thanthe diameter of Al2O3 inclusion in test 1. As shown inFig. 8(b) Ce2O2S inclusions appeared to have superiorresistance to pitting corrosion in test 4.

Both of the Al2O3 and Ce2O2S inclusions havestable chemical property and almost insoluble duringcorrosion test. But large Al2O3 inclusions are easy tobe the origin of pitting corrosion when theenvironment is full of Cl-. Fig. 2 indicates that thelarge inclusion distribution and inclusion average sizedecreased with the increasing content of Ce in thepresent results. Small size Ce2O2S inclusion has a lowcontract area with steel matrix, which would have abetter pitting resistance. Therefore, the addition of Cealloy to the spring steel led to a modification of largeAl2O3 changing to small Ce2O2S, which couldimprove the resistance to pitting corrosion.

For the purpose of improving the corrosionresistance and avoiding micro-cracks in spring steel, itwould be useful if the harmful Al2O3 inclusions couldbe wrapped or transformed by the rare earths. In thatcase, the rare earths inclusions can effectively

Q. Wang et al. / JMM 53 (3) B (2017) 365 - 372 370

[Ce] 6(Al O ) Al O Ce [Al]5

2 3 11 18

1

+ = ++5[Ce] Al O Ce 6AlCeO 5[Al]

411 18 3

++ = +

[

4[[Ce] 3[S] 2(AlCeO ) 3(Ce O S) 2[Al]3 2 2+ + = +

Figure 6. Schematic diagram of evolution process ofinclusions

Sample 1# 2# 4# 6#

Pitting potential -568 -513 -496 -485

Table 5. Chemical composition (wt %) of the spring steel

Figure 7. Schematic diagram of evolution process ofinclusions

suppress formation of cracks and voids in steel matrixaround the inclusions since the coefficient of thermalexpansion of rare earths inclusions is close to the steelmatrix [21] to avoid the additional force around theinclusions during hot rolling.

4. Conclusions

The evolution process of inclusions in spring steelused in fastener of high speed railway wasinvestigated. According to the experimental results,the following conclusions were obtained.

(1) Inclusions existing in Al-killed spring steel aremainly Al2O3 inclusions and clusters of Al2O3inclusions in the present study.

(2) With the increasing of Ce content in steel melt,Al2O3 inclusions were wrapped by rare earthinclusions to form a ring shape Ce-riched band aroundthe inclusion. All of inclusions in steel melt wouldcompletely modify to rare earth inclusions, when Cecontent is over 0.017%.

(3) The stability diagram of Ce-O and Ce-S at1873K are obtained by using FactSage6.4. It showsthat the inclusion variation isAl2O3→Al11O18Ce→AlCeO3→Ce2O2S in presentspring steel.

(4) The SEM-Mapping and line scanning resultsshow that Al2O3 inclusions could be transferred to rareearths inclusions. The inclusion is being changed fromAl2O3→Ce2O3-Al2O3→Ce2O2S.

(5) The modification of Al2O3 changing to Ce2O2Sin spring steel could improve the resistance to pittingcorrosion.

AcknowledgementThe authors would like to express their

appreciation to National Nature Science Foundationof China (No. 51734002，51374020), a FundamentalResearch Funds for the Central Universities (FRF-TP-15-052A3), Beijing Higher Education Young EliteTeacher Project (YETP0349) and China PostdoctoralScience Foundation (2014M560046).

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